The present application relates to a varactor element and an electronic device provided with the same, and more specifically, to a varactor element that changes the capacitance by applying a control field and an electronic device provided with the same.
In the past, varactor elements are utilized that apply a bias signal from outside and change the capacitance to control frequencies, time, and the like. As such varactor elements, varactor diodes (variable capacitance diodes) and MEMS (micro electro mechanical systems) are commercialized, for example. Such a varactor element used for these applications usually has two terminals and does not have a terminal exclusively for application of a controlling bias signal that controls the capacitance. Therefore, in an actual circuit, such two-terminal varactor elements are arranged to function as a four-terminal element.
FIGS. 27A and 27B illustrate an example of a circuit configuration when a two-terminal varactor element is arranged to function as a four-terminal element. In an example illustrated in FIG. 27A, a varactor element 160 (varactor capacitor) has one of the terminals connected to one of the input/output terminals of the alternating current signal via a bias removal capacitor 161 and also connected to an input terminal of the control voltage via a current limiting resistor 162. The varactor element 160 has the other terminal connected to the other input/output terminal of the alternating current signal and also connected to an output terminal of the control voltage.
In the circuit configuration illustrated in FIG. 27A, the signal current (alternating current signal) flows in the bias removal capacitor 161 and the varactor capacitor 160 and the control current (direct bias current) flows only in the varactor capacitor 160 via the current limiting resistor 162. At this point, by changing the control voltage, the capacitance of the varactor capacitor 160 is changed, and as a result, the signal current is also changed.
In an example illustrated in FIG. 27B, the varactor element 160 has one of the terminals, similar to FIG. 27A, connected to one of the input/output terminals of the alternating current signal via the bias removal capacitor 161 and also connected to an input terminal of the control voltage via the current limiting resistor 162. In the example illustrated in FIG. 27B, the varactor element 160 has the other terminal connected to the other input/output terminal of the alternating current signal via a bias removal capacitor 163 and also connected to an output terminal of the control voltage via a current limiting resistor 164. That is, in the example illustrated in FIG. 27B, the peripheral circuitry configuration connected to the formerly mentioned one terminal of the varactor element 160 is also applied to the other terminal.
In the circuit configuration illustrated in FIG. 27B, similar to the example illustrated in FIG. 27A, the signal current flows in the two bias removal capacitors 161 and 163 and also the varactor capacitor 160, and the control current flows only in the varactor capacitor 160. Therefore, in the example illustrated in FIG. 27B as well, the capacitance of the varactor capacitor 160 is also changed by changing the control voltage, and as a result, the signal current is also changed.
However in the circuit configuration illustrated in FIGS. 27A and 27B, while the control voltage source and the signal voltage source are configured individually, the terminal of the varactor element 160 to which they are finally connected is in common. In this case, although the two terminals are arranged to function as four terminals in the circuit, the direct bias current (control current) flowing from the control voltage source interferes with the signal current (refer to arrows in broken lines in FIG. 27B, for example).
Therefore, the circuit configuration illustrated in FIGS. 27A and 27B uses the current limiting resistor 162 and/or 164 for protection and/or separation of the control circuit, and uses the bias removal capacitor 161 and/or 163 for protection and/or separation of the signal circuit. The resistance value R is in particular established to be large for secure protection and/or separation of the control circuit. However, in this case, a time constant (=RC) determined by the resistance value R of the current limiting resistor 162 and/or 164 and the capacitance C of the varactor element 160 becomes large and the responsiveness of capacitance control is decreased.
Although the varactor elements illustrated in FIGS. 27A and 27B are provided with the bias removal capacitor(s) as they are substantially two-terminal elements, the present inventors already proposed a varactor element of a configuration without using such a bias removal capacitor (refer to Japanese Unexamined Patent Application Publication No. 2007-287996). In Japanese Unexamined Patent Application Publication No. 2007-287996, an element is proposed that uses a ferroelectric material as a varactor element. FIGS. 28A and 28B illustrate the electrode structure of a varactor element 200 proposed in Japanese Unexamined Patent Application Publication No. 2007-287996. FIG. 28A is a schematic perspective view of the varactor element 200, and FIG. 28B is a cross-sectional configuration diagram of a dielectric member 204 configuring the varactor element 200.
In the varactor element 200 according to Japanese Unexamined Patent Application Publication No. 2007-287996, respective terminals are provided on four faces of the dielectric member 204 in a rectangular parallelepiped shape. Out of the four terminals, two of the facing terminals are signal terminals 203a and 203b connected to a signal power source 203 and the other two facing terminals are control terminals 202a and 202b connected to a control power source 202.
Inside the varactor element 200, as illustrated in FIG. 28B, has a structure in which a plurality of control electrodes 202c through 202g and a plurality of signal electrodes 203c through 203f are alternately laminated via a ferroelectric layer 205. In the example of FIG. 28B, the control electrode 202g of the lowermost layer, the fifth control electrode 202e from the bottom, and the control electrodes 202c of the uppermost layer in the drawing are connected to one of the control terminals, 202a. The third control electrode 202f from the bottom and the seventh control electrode 202d from the bottom are connected to the other control terminal 202b. The fourth signal electrode 203e from the bottom and the eighth signal electrode 203c from the bottom are connected to one of the signal terminals, 203a. The second signal electrode 203f from the bottom and the sixth signal electrode 203d from the bottom are connected to the other signal terminal 203b. 
The varactor element 200 according to Japanese Unexamined Patent Application Publication No. 2007-287996 has a configuration of separately providing the control terminals and the signal terminals to separately apply the control voltage or the signal voltage to the respective terminals. The varactor element 200 according to Japanese Unexamined Patent Application Publication No. 2007-287996 also has a configuration in which the plurality of signal electrodes and control electrodes are laminated inside the dielectric member 204. Therefore, the varactor element 200 of Japanese Unexamined Patent Application Publication No. 2007-287996 has an advantage of allowing the capacitance to be increased at low costs. Further, the varactor element 200 of a structure such as in Japanese Unexamined Patent Application Publication No. 2007-287996 is manufactured easily at low costs. The varactor element 200 according to Japanese Unexamined Patent Application Publication No. 2007-287996 also has an advantage of working without a bias removal capacitor.